104Implement asyncMap() over an async iterable with bounded concurrency and backpressure

Implement an async generator asyncMap(source, mapper, { concurrency = 4, ordered = true } = {}). source is any iterable or async iterable (for example paginated API results or lines of a huge file). It yields await mapper(item, index) for every item.

for await (const thumbnail of asyncMap(readFiles(folder), makeThumbnail, { concurrency: 3 })) {
  gallery.append(thumbnail);
}

Requirements

• At most concurrency mapper calls run at the same time.
• Backpressure: pull from source only when there is room. If the consumer stops reading, the generator stops pulling. The number of items pulled but not yet yielded never exceeds concurrency.
• ordered: true yields in input order; ordered: false yields each result as soon as it is ready.
• If the consumer stops early (break in for await), close the source (call its return(), so its finally blocks run).
• If a mapper rejects or the source throws, the generator throws that error.